1. Field of the Invention
This device relates to a signal routing device, and more particularly to such a device for routing a signal selectably to any one of a set of outputs.
2. Discussion of Prior Art
Signal routing devices such as multiway switches are well known for use at DC and conventional electronic frequencies. However, related devices for use at visible, infra-red and microwave frequencies are much more difficult to implement. One such device is disclosed in Patent Co-operation Treaty Application Number PCT/GB88/00928 published as W089/04988 on Jun. 1st, 1989. It relates to a phased array of electro-optic waveguides. It produces an output beam (main diffraction lobe) which is steerable across a set of outputs by varying bias voltages applied to individual waveguides. The mode of operation employs radar phased array principles. This device is unfortunately characterised by appreciable radiation losses and consequent inefficiency. It is necessary to provide inputs to each of the waveguides, which involves flooding a common input region with radiation. Much of this radiation does not pass down any waveguide. Moreover, the phased array output includes unwanted sidelobes (subsidiary diffraction maxima) corresponding to wasted signal.
An optical gate matrix switch capable of switching any input line to any output line is described by A Himeno, H Terui and M Kobayashi in "Guided Wave Optical Gate Matrix Switch", Journal of Lightwave Technology, Vol. 6, No 1, (1988) pages 30-35. The matrix switch is constructed by integrating InGaAsP laser diode gates with high silica guided wave splitter and recombiner circuits. It is designed for operation with light of wavelength 1.35 .mu.m. The four input, four output device described has overall dimensions of 10 mm by 25 mm, and an estimated loss of up to 22.6 dB. It therefore suffers from the following disadvantages: a relatively high level of complexity, large size and high optical loss. Indeed amplifiers in the output lines are described as necessary for most potential applications.
A further waveguide signal routing device is described by R Ulrich in UK Patent 1 525 492. The device incorporates q input waveguides feeding a first multimode self-imaging waveguide, which is connected, via q multimode relay waveguides to a second multimode self-imaging waveguide, which has q output guides connected to it. Here q is a positive integer. The self imaging waveguides are of rectangular cross-section and provide for energy efficient beam division and recombination. The relay waveguides are self-imaging, are of square cross-section and contain phase shifters.
Radiation input via one of the input waveguides is, by virtue of modal dispersion within the first self-imaging waveguide, directed to the q relay waveguides. Each relay waveguide receives 1/q of the input radiation. The phase shifters are employed to alter the relative phases of the radiation in the relay waveguides. The phases are altered such that when the radiation passes to the second self-imaging waveguide it is, by virtue of modal dispersion recombined and directed to a chosen one of the output waveguides.
For the device to operate as described above the first and second self-imaging waveguides and the relay waveguides must be of appropriate lengths. The lengths are given by the equation ##EQU1## where: h is a parameter which varies with the function of the waveguide, as discussed below;
W is the transverse width of the waveguide; and PA1 .lambda. is the wavelength of the radiation in the waveguide. PA1 (a) a multimode waveguide arranged to divide the intensity of radiation input to it between each of a set of relay waveguides, PA1 (b) phase shifting means arranged to provide for radiation within each relay waveguide to have variable phase relative to that of radiation within the other relay waveguides in each case, and PA1 (c) radiation redirecting means arranged to provide for radiation within the relay waveguides to become redistributed, PA1 characterised in that
For the relay waveguides h is an integer. For the first and second self-imaging waveguides h is given by ##EQU2## respectively, where q is the degree of splitting or recombination as appropriate; q does not equal 1, and p.sub.1 and p.sub.2 have integral values which are prime relative to q.
The prior art device described in UK Patent No. 1525 492 therefore suffers from the disadvantage that its length is constrained. That is the first and second self-imaging waveguides and the relay waveguides have to be of particular lengths and the device therefore has a minimum length beyond which it cannot be reduced. In addition, in order to operate as described, the device requires of the order of fifty waveguide modes to be supported within it. The production of waveguides capable of supporting such a large number of modes is both difficult and expensive.